1. Field of the Invention
The present invention relates to an optical recording medium and a drive of such optical recording medium, and more particularly to a preformatted portion, in which preformat information such as ID information is recorded, of an optical recording medium employing a land/groove recording system enabling recording at high density by varying the height of adjacent tracks.
2. Description of the Prior Art
Recently, as the computer becomes higher in performance, there is an increasing demand for memories of larger capacity, and optical disks and magneto-optical disks are developed. Toward the forthcoming age of multimedia, a further larger capacity is required. Optical disks include the read-only type, write-once type, and multiple recordable type such as magneto-optical disk.
Recordable optical disks are classified into a type of using a phase changing material in the recording film, and a type of using magneto-optical material. The phase changing material induces phase shift between, for example, crystal and amorphous, becoming crystal and raised in light reflectivity when exceeding a certain temperature T1, and becoming amorphous and lowered in light reflectivity when exceeding a certain temperature T2. Such reflectivity changes are made to correspond to "0" and "1", and recording and reproduction are executed. When recording in such optical disk, using a beam focused to about 1 .mu.m, the temperature of the optical disk surface is raised until phase change takes place. When erasing, by lowering to a temperature below the recording point, it is returned to the state before recording. When reproducing this optical disk, by illuminating with light sufficiently weaker than in recording and erasing, "0" and "1" can be read out from the difference in light reflectivity between recorded portion and erased portion.
The magneto-optical material forms a vertical magnetized film on a substrate, and is initialized by first applying an external magnetic field in one direction. When recording, on its surface, an external magnetic field is applied in the opposite direction of initialization, and the surface is heated by a beam focused to about 1 .mu.m nearly to the Curie temperature to decrease the coercive force of the surface portion, and the magnetization is inverted to a direction of the external magnetic field. When erasing, the external magnetic field is set in a same direction as in initializing. When reproducing, a polarized beam is emitted to the disk, and the direction of Kerr rotation by the magnetization direction of magnetic film is read out by an optical system for detecting polarization such as an analyzer.
In a conventional optical recording medium, as shown in FIG. 1, a convex track (land) 3 divided by a V-section groove 1 is a record region of pits 4 for expressing record information, and this track is divided into sectors of predetermined length, and an ID signal expressing the position information is recorded at the beginning of each sector in an array of prepits 2.
To enhance density in such conventional optical recording medium, hitherto, a land/groove recording system has been proposed for widening the groove used in division of track and recording information also therein. When the ID signal is formed in prepits in this method as in the prior art, since the interval of adjacent tracks is narrow, an adjacent signal often mixes in to cause crosstalk, and the ID signal cannot be reproduced correctly.
Accordingly, various ID signal recording and reproducing methods have been proposed for the land/groove recording system, but when such recording and reproducing methods are realized, the speed detection of optical head often goes wrong.
FIG. 2 (a) is a top view of an example of an ID signal recording and reproducing system involving the possibility of error in speed detection. In this optical recording medium, a row of prepits P2 for ID signal is arranged at an intermediate position in the radial direction between the concave groove track G and convex land track L, and the row of prepits P2 for ID signal is shared between the land track L and groove track G on both sides. In this ID signal recording and reproducing system, a groove (not shown) penetrating through the row of prepits P2 is provided so that error may not be caused in track counting when the optical head moves between the tracks.
In this system, when the optical head moves from A to C between the tracks, as the optical head moves from the recording/reproducing region including the groove G to the ID information region including the row of prepits P2 or from the ID information region to the recording/reproducing region, as shown in FIG. 2 (b), the track error signal indicated by dotted line is deviated in phase by 90.degree. as indicated by solid line, and the binary signal of the tracking error which is intended to be inverted at an intermediate position in the track direction of the ID information region as shown in FIG. 2 (c) is inverted at a different interval from the intended inverting time interval as shown in FIG. 2 (d), and an error is caused in track counting. Speed control of the optical head when tracking a specified track at the time of seeking is done during the time of crossing one track from inversion to next inversion of the binary signal of the tracking error signal, and the demerit is that the target track cannot be determined accurately.
FIG. 3 is a top view of another example of ID signal recording and reproducing system involving error in speed detection. In this optical recording medium, a concave groove track G and a convex land track L individually include a row of prepits P3 for ID signal. However, to prevent crosstalk between rows of prepits P3 as mentioned above, a row of prepits P3 of groove track G is arranged in the first half of the ID information region, and a row of prepits P3 of land track L in the latter half. In this ID signal recording and reproducing system, in order to prevent error in track count when the optical head moves between the tracks, a groove G1 penetrating through the row of prepits P3 is provided.
In this system, since the row of prepits P3 for land track L is positioned in the middle (180.degree.) of the adjacent groove tracks G, the tracking error signal produced from the row of prepits P3 for land track L is deviated by 180.degree. in phase from the tracking error occurring elsewhere, and an error is involved in the track counting.